Fixed bugs affecting long stackings stability

Changes since s2.02:

 - I forgot to include help.ino in past releases, now it's included.

 - Modified the included library files Keypad.* to make keypad scans impulse noise-resistant. Now key scan has to return
   bit-identical map N_KEY_READS (=3) times in a row to have the change accepted. This fixed the occasional
   fake key triggering during long (>20 minutes) focus stacks.

 - Upgraded all float types to real64 (from real32). Also introduced LONG_TIME macro option in stacker.h; when
   defined, all time related variables get 64-bit integer type. Introduced my own 64-bit version of micros(), in micros_my().
   All these changes are critical to get full stability for long (>20 minutes) continuous stacking shootings.

 - Replaced roundMy() with two functions - round_coords() and round_time(), as they deal with different types.

 - Modified TIMING macro option behaviour. Now it measures the average Arduino loop length, and its std, for the
   most recent move. It is displayed at the left bottom corner when pressing the "*" key. The units are
   microseconds.

New Burst shooting mode, base acceleration can now be adjusted

Version s2.02.

New features:

• Base acceleration (the one used for all moves except for the initial rewind/fast-forward acceleration, and emergency breaking) can now be adjusted in the UI, via *9 command (parameter Acc2). It has the same scale as the Acc parameter (*A) - the larger the number, the lower is the acceleration. Can be important if your camera is heavy (say >1kg), to minimize the force on the rail threads. Also you might need it if you get too much vibrations every time rail stops moving.
• New "Burst" mode of camera operation has been added (*C command) - "Brst". Can only be used in 1-point shooting. In this mode, the camera shutter and AF are pressed for the whole duration of the stack. To be used with a burst shooting mode present in most cameras. Frame counting and steps between frames are now approximate. For many cameras, this mode will allow for much larger frames per second shooting, compared to other Fast Stacker modes where we explicitly press shutter for each frame. Usually there is a limit on how many shots can be made in a burst mode (17 RAW images for Canon 6D, at 4.5 fps).

Fixes / small improvements:

• To accommodate the new features, one memory register was removed (so it's down to 5 memory registers).
• Now live position is constantly updated - as long as the speed is small enough (less than POS_SPEED_FRACTION = 0.1 fraction of the maximum speed; parameter in stacker.h). There is a limit because at higher speeds rail movements become much noisier (with more vibrations), as updating the position interferes with motor operation.
• The values for *X parameters now use descriptive language instead of numbers.
• Foreground/Background points are now detected/displayed more reliably.
• Now displaying AC bitmap when powered via external AC adapter (when per-battery voltage is above some critical value, V_THRESHOLD = 11.3 / 8 Volts).
• Small bug fixes.

s2.01

A few fixes and improvements

First public release of Fast Stacker 2.0

Hardware and software are completely redone. New microprocessor, display etc. 3D printed enclosure, factory-manufactured PCB. Much faster, much more accurate, much better UI, extreme macros up to 60:1 are now possible. The project wiki pages (https://pulsar124.fandom.com/wiki/Fast_Stacker) will be gradually updated to reflect the changes.